This invention relates to an apparatus for injecting plastics, elastomers, or the like, particularly intended for reinforced plastics such as thermosettable materials containing glass fibers.
In known injection devices the material is brought by means of a feed device, generally funnel-shaped, to a screw located in a fixed sheath where it is mixed and then injected into a mold by the movement of the screw within its sheath. To prevent the mixed material from returning into the screw during the injection phase, the end of the screw is formed as a valve which closes automatically during injection. This has the disadvantage that during the mixing of the material by the screw, the valve forces the material to follow a winding path which can destroy the properties of the material, in particular the breakage of the glass fibers such that the final molded article has limited mechanical properties.
In other known devices this disadvantage is cured by placing the screw in a movable sheath which supports the material feed device and which is mounted to slide within a fixed sheath. This movable sheath, which acts as an injection piston, forms a variable volume chamber together with the fixed sheath for receiving the material to be injected. During such injection this movable sheath, which also acts as a valve with the end of the screw, prevents the material from returning back into the screw.
Since with these prior art constructions the fixed sheath is always connected directly to the mold, the mixed plastics material contained in the sheath prior to its injection is subjected to the influence of the mold temperature. Thus, in the injection of thermoplastic materials where the mold is cooled, the fixed sheath is also cooled on contact with the mold whereby the plastics material becomes hardened and blocks the injection orifice. On the other hand, in the injection of thermosettable materials where the mold is heated to ensure the polymerization of the material, the fixed sheath is also heated so that the material begins to polymerize in the sheath before its injection.
Such undesired thermal transfer can profoundly alter the plastics material when it is brought into the variable volume chamber by the mixing screw, particularly when the chamber is supplied directly with the material by its own weight. In effect, a shearing of this material takes place between the threads of the screw and the internal wall of the movable sheath. When the plastics material additionally contains glass fibers, this shearing effect is particularly onerous since it results in the breakage of the fibers and the subsequent mechanical weakening of the finished molded product. In other known injection devices an Archimedes screw is used to ensure a forced supply of the plastics material, but this does not decrease the shearing of the material in the screw. Further, when the injection apparatus itself does not have a mixing screw, the supply of the plastics material by an Archimedes screw does not allow the variable volume chamber to become filled.